By Bertha Hu"With bread yeast and humans, there are about 45 amino acids that are different and about 59 that are identical. Think how close together man and this other organism, bread yeast, are. What is the probability that in 59 positions the same choice out of 20 possibilities would have been made by accident? It is impossibly small... The developmental explanation is that bread yeast and man have a common ancestor, perhaps two billion years ago. And we see that not only are all men brothers, but men and yeast cells, too, are at least close cousins." -Linus Pauling, The Science Teacher (1933)

Classification and Diagnostic Characteristics

Domain

Eukaryota

Kingdom

Fungi

Phylum

Ascomycota

Class

Saccharomycetes

Order

Saccharomycetales

Family

Saccharomycetaceae

Genus

Saccharomyces

Species

Saccharomyces cerevisiae

[2]Yeasts are eukaryotic cells under the kingdom Fungi. Most are unicellular, although some species may have multicellular life stages. Yeasts have typical eukaryotic structures and have membrane-enclosed organelles. They are facultative anaerobes, or organisms that can grow with or without oxygen, and they obtain nutrients directly across their cell surfaces. Yeasts live in a variety of environments, most of which are moist, and they have thick polysaccharide cell walls. Yeast cells range from 1-5 µm wide by 5-30 µm long. [4] They can live as either haploid or diploid, depending on the environmental conditions. Under adverse conditions, for example, the diploid cells will undergo meiosis and sporulation. Additionally, yeasts convert carbohydrates to carbon dioxide and alcohols, aiding in the making of liquor and bread. Some yeasts are also pathogens, causing infections in people usually with weakened immune systems. “Yeast” does not refer to a single taxonomic or phylogenetic group, but rather to a lifestyle that has evolved multiple times. Although there are over 1,500 species of yeasts, the term is usually synymous with the well-known Saccharomyces cerevisiae, a 16 linear chromosome, 5,770 gene organism that has been key in wine making, brewing, and baking since ancient times. [1]

Relationship to Humans

Because they are so easy to cultivate, because of their rapid growth rates, and because their characteristics encompass a larger group of similar cells, yeasts have become one of the most thoroughly researched eukaryotic microorganisms. They have many of the same advantages to culture as do bacteria, but yeasts are eukaryotes and thus have genome structures much more like those of humans. Furthermore, some yeasts such as Saccharomyces are commonly used as eukaryotic hosts for recombinant DNA studies because of the yeasts’ rapid cell division, ease of growth in labs, and relatively small genome size. Yeasts are model organisms for study and are incredibly important in modern cell biology research. [1] Moreover, the yeast genome contains over 12 million base pairs, or about 6,000 genes. 20% of human disease genes have counterparts in yeast, suggesting that these diseases result from disruption of DNA repair, cell division, or control of gene expression. Additionally, genes found in yeast are used to test new drugs, and certain chemicals that restore yeast cells back to normal functioning are being tested to confirm if they also help humans. [5]Yeasts are also important for making beverages and bread. They have been used throughout history for food production. Because they are capable of alcoholic fermentation, yeasts are pivotal for the production of liquor. Alcoholic beverages contain ethanol, a byproduct produced by yeast fermentation. Yeast is necessary for creating beverages from wine and beer to vodka and mead. Furthermore, because yeasts also produce carbon dioxide, they are excellent leavening agents. Fermentable sugars in dough are converted into carbon dioxide by yeasts, causing bread to rise as the gas forms air pockets, producing a fluffy quality. [1]

Yeast cells are important in making bread and wine.

When a person has weakened immunities, he or she could get a yeast infection. People with AIDS, for example, often die from pneumonia, inflammation of the lungs caused by Pneumocystis jirovecii. Moreover, yeasts like Candida albicans cause diseases like esophagitis, in which the esophagus is inflamed, irritated, and swelled, impairing one’s ability to swallow. Other fungi can cause less threatening conditions such as ringworm and athlete’s foot. Pathogenic fungi also greatly affect animals and plants. At the moment, knowledge on fungal illnesses is quite limited, so many diseases are still unable to be treated. [1]

Habitat and Niche

Yeasts are extremely common in the environment. They can be found on fruit skins, plant saps, and soils all the way to human toes, mammal digestive tracts, and the deep sea. Most yeasts grow best under neutral pH. The temperature range at which yeasts thrive in varies greatly depending on the species. [1] In regards to Saccharomyces cerevisiae, yeast cells of this species can be found in decaying fruits like grapes and the insides of animals, plants, and other sugary foods, along with in grocery stores, stored as dry yeast. Dry yeast are inactive due to lack of moisture. [6]Yeast cells are important providers of carbon dioxide, produced when they break down sugars. [1]

Predator Avoidance

Yeasts, lacking the ability to move, have no specific predatory avoidance tactics against enemy organisms. [1] They do, however, have a very simple defense mechanism. Yeasts have chitin-rich cell walls along with lipid bilayers covered in proteins and cellulose that surround the cell membrane. The protein covering, composed of many different layers that serve to protect the yeast from invaders, allows for defense against foreign materials. [7] Furthermore, because yeasts like many other fungi can develop in a variety of places and proliferate quickly, they are high in number, so predation is not an overwhelming problem.

Nutrient Acquisition

Yeasts absorb nutrients directly across their cell surfaces. As chemoorganotrophs, they oxidize chemical bonds in organic compounds such as sugars, fats, and proteins, as their energy source. Yeasts obtain carbon mainly from glucose, fructose, sucrose, and maltose. Enzymes inside the mitochondria of yeasts break up the food molecules, releasing energy for fueling the cell while making waste products. In the presence of air, yeast cells produce carbon dioxide through oxidative phosphorylation. Under anaerobic conditions, however, yeasts use the irreversible process of alcoholic fermentation, in which pyruvate is converted into acetaldehyde and carbon dioxide is released. NADH, a high energy molecule used by cells to generate ATP, is used afterwards to reduce acetaldehyde into ethanol. Ultimately, yeasts convert sugars and starches into carbon dioxide and ethanol. [1]

The process of alcoholic fermentation.

Reproduction and Life Cycle

A budding yeast cell.

Like all fungi, yeasts may have asexual and sexual reproductive cycles. Environmental conditions often determine the mode of reproduction that takes place at any given time. Yeasts are capable of asexually reproducing by budding or sexually reproducing by sporulation. Yeasts can exist as either a diploid or haploid and mainly reproduce by mitosis, with daughter cells budding off of mother cells. The nucleus of the parent cell splits and migrates towards the daughter cell. The bud continually grows until it can separate from the parent cell and function independently. Diploid cells, usually when under stressful conditions, can undergo meiosis and sporulation to produce four haploid spores. [1] Beginning with plasmogamy, or the fusion of the cytoplasm, the cell becomes fertilized and diploid. The diploid zygote then forms, and meiosis follows to produce haploid cells. With adequate nutrition, Saccharomyces cerevisiae can usually reproduce every 100 minutes. The average replicative lifespan is about 26 cell divisions. [3]

Growth and Development

Budding yeast can live as either diploids or haploids, having two genomes or one genome respectively. Haploid yeasts occur in the two mating types a or α, determined by gene expression at an active mating type locus. They can live indefinitely as haploids, but if two cells of opposite mating types meet (a and α), they can fuse and enter the diploid phase of the cell cycle. Diploid yeasts are more resistant to harsh environmental conditions. They can undergo meiosis when food is scarce, creating four haploid spores, two a and two α. [1] The growth rate of yeast cells depends largely on the availability of nutrients and sugars. Under non-ideal conditions, yeast cells can adjust the length of their cell cycle, thus affecting overall growth rate. Overall, yeasts grow quite slowly and do not become any bigger than parent cells. [8]

Integument

The yeast’s cytoplasm is protected by a cell wall and membrane. Yeasts contain chitin-rich cell walls, mainly comprised of polysaccharides made up of glucose, mannose, and N-acetylglucosamine. [9] The membrane is comprised of lipids with hydrophilic heads and hydrophobic tails. Organelles, specialized subunits within cells, are also enclosed with a membrane. [1]

Movement

Yeast cells are immotile, so they can only move passively by means of air or water currents. They lack flagella and other cell mechanisms for movement. [1] When sexually reproducing, however, yeast cells frequently produce a mating projection, resulting in elongated cells. This behavior, coined "shmooing," produces a nodule that yeast cells use to join together. The process of shmooing takes about two hours.

Sensing the Environment

When yeasts are put in stressful situations, such as nutrient starvation, diploid cells can respond by sporulating, entering meiosis and producing haploid spores. [1]

Gas Exchange

When yeast cells release carbon dioxide as a waste product, the gas is diffused through the cellular membrane. Furthermore, with aerobic yeasts, the porous surface also allows oxygen to enter the cell and be used for fermentation. Yeasts rely on their tiny spores throughout their bodies for aerobic respiration. [1]

Waste Removal

When yeast metabolizes sugar, it produces carbon dioxide gas and sometimes ethanol alcohol as waste products. The fungus packs these unwanted molecules into vacuoles, organelles that serve secretory, excretory, and storage functions, and releases them through the cell membrane. [1]

Environmental Physiology

Yeast cells can respire normally when oxygen is present and use alcoholic fermentation under anaerobic conditions. [1] Additionally, depending on the species of yeast, they can tolerate extreme temperatures. In general, yeast cells can survive between 0C to 10C, but cannot grow. Yeast cells develop very rapidly during the optimal temperatures of 10C to 37C. After surpassing 37C, the yeast becomes less and less efficient until around 50C, where its structure denatures. [12]

Internal Circulation

Because they are unicellular, yeasts have no internal circulatory systems. Single-celled organisms are small enough so that materials can easily move within the cell without a special system. Their selectively permeable membranes allow them to absorb starchy nutrients directly through their cell surfaces. Yeasts are capable of controlling wastes in their systems by filling vacuoles with undesired molecules and expelling them. Vacuoles are also necessary for homeostasis maintenance, osmoregulation, and amino acid storage. [1]

Chemical Control

The cell wall and membrane protect the yeast cell from the external environment. Membrane receptors can receive signals and spark change within the cell body. [1]

Review Questions

Explain how yeast cells are able to absorb and get rid of nutrients without any internal circulation.

How does yeast react to stressful environments?

Explain the process of fermentation. Why is this process considered anaerobic?

Explain the yeast life cycle in terms of budding and sporulation.

How do yeast protect themselves from predators?

Explain what yeast attributes are used in baking.

What biological mechanisms do yeast use to withstand such a large range of temperatures?

Why are yeast cells found in such high numbers, and why is this integral to their survival?

Explain the basics of the glucose-sensing mechanism and how it better enables yeast to adapt to different types of environments.

Under what temperatures will yeast perform optimally?

References

Hillis, David M., David Sadava, H. C. Heller, and Mary V. Price. Principles of Life. High School ed. Sunderland: Sinauer Associates, 2012. Print.